Combining Diffusion NMR and Small-Angle Neutron Scattering Enables Precise Measurements of Polymer Chain Compression in a Crowded Environment

Abstract

The effect of particles on the behavior of polymers in solution is important in a number of important phenomena such as the effect of "crowding" proteins in cells, colloid-polymer mixtures, and nanoparticle "fillers" in polymer solutions and melts. In this Letter, we study the effect of spherical inert nanoparticles (which we refer to as "crowders") on the diffusion coefficient and radius of gyration of polymers in solution using pulsed-field-gradient NMR and small-angle neutron scattering (SANS), respectively. The diffusion coefficients exhibit a plateau below a characteristic polymer concentration, which we identify as the overlap threshold concentration c ⋆ . Above c ⋆ , in a crossover region between the dilute and semidilute regimes, the (long-time) self-diffusion coefficients are found, universally, to decrease exponentially with polymer concentration at all crowder packing fractions, consistent with a structural basis for the long-time dynamics. The radius of gyration obtained from SANS in the crossover regime changes linearly with an increase in polymer concentration, and must be extrapolated to c ⋆ in order to obtain the radius of gyration of an individual polymer chain. When the polymer radius of gyration and crowder size are comparable, the polymer size is very weakly affected by the presence of crowders, consistent with recent computer simulations. There is significant chain compression, however, when the crowder size is much smaller than the polymer radius gyration.

The effect of crowder charge in a model polymer–colloid system for macromolecular crowding: Polymer structure and dynamics

“…We can use the fitted results to recast all the measurements of polymer self-diffusion in charged Ficoll70 (colored symbols), as well as the previous results with uncharged As discussed in Sec. II and in previous work in the presence of an uncharged crowder, 21 such an exponential relationship could be consistent with the theory for atomic liquids where an exponential relationship between atomic diffusion and the excess entropy is predicted. 33,34 Such a remarkably universal exponential dependence at all Φ F , and independent of crowder charge, suggests that above c , only the structure is important, and colloidal hydrodynamics is unimportant.…”

“…In a recent study, 21 we have shown that the uncharged Ficoll70 crowder induces little compression in the polymer, consistent with a simulation that assumes hard-sphere crowders; 22 this suggests that Ficoll70 does not associate, and is thus an inert crowder, at least with respect to PEG. However, it is unclear whether an uncharged, hard-sphere crowder is relevant to real biophysical situations, such as macromolecular crowding in living cells where the macromolecules are charged entities, such as proteins and nucleic acids.…”

“…5(b)] the polymer size in the polymer-dilute regime: R g (0, Φ F ). For uncharged crowders, results from previous work 21 show a weak dependence on Φ F . For charged crowders, there is a ≈ 30%-40% increase in R g in both cases, which is significant.…”

“…In previous work on polymer dynamics in the presence of uncharged crowders, 21 we discovered that the polymer self-diffusion coefficient exhibits a sharp change from a polymer-concentration independent dilute regime [with a plateau value D(0, Φ F )] to a crossover regime above a concentration c where D(c p , Φ F ) could be fitted with an exponential dependence on c p . This is expressed in the piecewise function…”

Small-Angle Scattering from Fractals: Differentiating between Various Types of Structures

“…With the recent advances of its derivative techniques, i.e., grazing-incidence [3], anomalous [4], scanning [5] or magnetic [6] SAS, the range of applications has been greatly extended. This includes now a much larger class of hierarchical materials, i.e., materials in which the structural elements themselves have a structure, such as biological macromolecules [1,[7][8][9], polymers [10][11][12][13], composites [14][15][16][17] or cellular solids [18][19][20].…”